Holes having both a straight wall portion and a tapered wall portion are commonly used in the aircraft industry and other industries where fasteners such as rivets are used to attach an outer skin to a frame. This is particularly true when aerodynamic requirements specify low turbulence and low drag across the surface of the skin being attached to the frame. When the body being fabricated must meet specifacations of low turbulence and low drag it is not enough to merely provide a fastener hole in the skin that is couter-sunk (a hole having portions wherein the side walls are straight and portions with tapered sidewalls) but the countersinking must be down to a precise depth and concentric with the straight wall of the hole so that the head of a fastener used in the hole will be exactly flush with the surface of the skin. If such is not the case, turbulence will be induced by the uneven skin surface as will drag because of the rough skin surface. In the past the absolute depth of the countersunk portion of a hole was not measured but instead fasteners were placed in holes and one by one the relative flushness of each fastener and the surface of the skin was measured. The device used to measure fastener flushness in this manner is expensive and the practice of the method is time consuming and costly because it is so labor-intensive.
Thus, it can readily be seen that the prior art methods of and apparatus for measuring the depth of the countersink portion of countersunk holes do not measure actual depth of the tapered portion of the hole and leave much to be desired both in terms of accuracy and efficiency. Disclosure of the Invention
Accordingly, it is an object of the present invention to provide a gauge that can be used to measure directly and accurately the depth of the tapered portion of a countersunk hole.
It is also an object of the present invention to provide a gauge capable of measuring relative flushness of a countersunk hole directly and without requiring that a fastener be placed in the hole before such a measurement can be made.
The principal feature of the present invention is the provision of a unique selfcentering gauge for determining the dimensions of the tapered portion of a couatersunk hole which can be used with holes of various diameters. In accordance with the present invention a measuring device is provided which when used automatically centers itself in the countersunk hole being measured and can alternatively display either the actual depth to the countersunk portion of the hole or the relative flushness of a series of holes as compared to a standard depending on how zero setting is made. The measuring device is in the form of a hollow body which has attached to one end of a stem gauge, the stem portion of which extends into the hollow body and therein contacts one end of a probe, the other end of which forms a measuring surface. The probe is held inside the hollow body by a foot piece which is adapted to contact an annular ring which surrounds the measuring surface at its juncture with the main body of the probe. The probe is forced against the foot piece by a spring which surrounds the probe and an inner sleeve which surrounds the probe concentrically with slip fit and also serves as a part of the mechanism by which the stem gauge attaches to the hollow body. The slip fit between probe and inner sleeve eliminates the possibility of erroneous reading. An expandable centering device is attached to the apex of a conical measuring surface so that the measuring surface of the gauge is always positioned in the center of the countersunk portion of the hole being inspected.
An additional feature of the present invention is the ability of the gauge to be used to inspect holes of various diameters with equal accuracy without changing either the centralizing device or the probe measuring surface.
Another feature of the present invention is the ability of the measuring surface to automatically position itself so that contact between the gauge and the hole always occurs at the intersection of the tapered walls of the countersink and the straight walls of the hole.
Still another feature of the present invention is the ability of the gauge to determine and display either actual depth of the countersink or comparative depths based upon a calibration standard.
In accordance with the present invention the counter-sink measuring device comprises a hollow body of a stem gauge attached to one end of the hollow body so that the stem of the gauge extends into the exterior of the hollow body via an inner sleeve which houses the plunger position of the gauge and a probe. One end of the probe comprises a measuring surface in the shape of a truncated cone and which has been adapted to function as a collect holder which receives and holds the centralizer. The other end of the probe functions as a contact point for the stem gauge plunger and is held against the gauge plunger by a spring which surrounds the inner sleeve and the part of the probe extending therefrom. An annular ring machined into the measuring surface at the base of the truncated cone contacts a foot piece which threads onto the hollow body and which holds the probe and spring inside the hollow body. The foot piece contains a central aperture through which the measuring surface of the probe and the centralizer project so that the measuring surface and centralizer can be pressed into a hole so that the measuring surface always contacts the hole at the junction of the tapered walls of the countersink with the straight wall section of the hole.
In accordance with the present invention the measuring device also includes means for limiting the lateral movement of the measuring surface within the countersink.
Also in accordance with the present invention the centralizer comprises an elongated collect mounted on the probe and coaxial with the long axis thereof and which includes an expandable spherical member extending from the center of said measuring surface in combination with a mandrel, wedge shaped on one end and threaded into said collet so that as the mandrel is threaded into and out of the collet the wedge shaped end causes the spherical member to expand and contract respectively and further includes a means for firmly locking the collet and mandrel in the collet holder.
Also, in accordance with the present invention a foot is provided which allows the measuring device to be used to measure countersink depth when a template is mounted on a piece part without removing the template. If there is a discrepancy in depth when measured, necessary corrective action can be taken to improve the quality of holes, i.e. the adjustment of power or hand tool to control the depth of countersunk hole.
In accordance with the present invention the included angle of the conical measuring surface is less than the angle of taper of the countersunk portion of the hole so that contact between the measuring surface and the hole only occurs at the intersection of the vertical and tapered portions of the hole. The included angle of the conical measuring surface generally is from about 50 degrees to about 110 degrees, however, optional angle would be around 55 to 60 degrees if the hole is through and there is no obstruction at the back side of the piece part. In the aircraft industry the included angle will generally be less than 100 degrees.
In accordance with the present invention the measuring surface comprises a fluted truncated cone adapted to receive a centralizing member.
Also in accordance with the present invention the stem gauge includes an analog display means for displaying either the actual depth of the tapered portion of a hole or the comparative depth of the tapered portions of several holes as they compare to a standard,